An aqueous hydroxylamine solution has been hitherto used, for instance, as a processing solution or detergent for producing electronic devices. In particular, a highly-concentrated and highly-purified aqueous hydroxylamine solution containing almost no impurities (particularly, metal components) has been recently required, for example, for washing very small and highly-integrated electronic devices such as semiconductor IC chips.
Hydroxylamine (NH.sub.2 OH) is generally synthesized by a known method. In the method, sodium bisulfite and sodium nitrite are reacted in an aqueous phase to prepare an aqueous solution of sodium hydroxylamine sulfonate, which is then hydrolyzed to obtain an aqueous hydroxylamine sulfate solution. The obtained solution is neutralized with sodium hydroxide to give an aqueous free hydroxylamine solution.
Hydroxylamine normally takes the form of crystals at room temperature, and its melting and boiling points are 33.degree. C. and 57.degree. C. (at 20 mmHg), respectively. The crystals of hydroxylamine are known to be explosive when heated, and an aqueous hydroxylamine solution is also known to be so unstable that it is liable to rapidly decompose. For this reason, an aqueous solution of stable hydroxylamine salt is beforehand prepared, and the free hydroxylamine is produced when it is used in industry. Since this process is troublesome and inconvenient, it has been desired to find out a stabilizing agent specifically effective for free hydroxylamine. Examples of the stabilizing agents hitherto having been proposed include 8-hydroxyquinoline (Japanese Patent Provisional Publication No. 57-100908), 1,10-phenanthroline (Japanese Patent Provisional Publication No. 58-69841), bipyridine (Japanese Patent Provisional Publication No. 58-69842), thiocarboxylic acids (Japanese Patent Provisional Publication No. 58-69843) and quinoline (Japanese Patent Provisional Publication No. 58-69844). Further, WO 97/22549 and U.S. Pat. No. 5,783,161 teach that trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid effectively stabilizes free hydroxylamine.
The already proposed stabilizing agents, however, are not fully effective when used in a normal manner. In addition, a prepared aqueous hydroxylamine solution generally contains impurities such as metal components (e.g., iron, aluminum, alkali metals such as sodium and potassium), which are originally contained in the starting materials or introduced in the preparation process.
In order to prepare a highly-purified aqueous free hydroxylamine solution suitable for processing or washing semiconductor devices, concentration and distillation by heating are indispensable. However, these treatments are liable to cause explosion, and hence it is very difficult to safely distill the aqueous hydroxylamine solution (which is unstable and corrosive) keeping its distillate from contamination with impurities.